An opposition leader in Ethiopia has demanded that "political prisoners" be freed as anti-government protests continued to rage in one of Africa's most populous countries.

The call was made by Tiruneh Gamta, a leader of the Oromo ethnic group, from which the biggest number of protesters come.

According to the New York-based Human Rights Watch, at least 500 people have been killed since unrest began in November.

Thousands have also been arrested, rights groups say, and many have not been heard from since they were detained.

"We want all political prisoners, regardless of any political stand or religion or creed, released from jail. Together with this, we need democratic rights," Gamta told Al Jazeera.

The government has denied that violence from the security forces is "systemic" and pledged to launch an independent investigation, blaming opposition groups inside and outside of the country and what it called "anti-peace" elements for the chaos.

Al Jazeera interviewed a woman who said she was arrested while on her way to a market. A protest had been taking place close to the market, she said.

After a night in a jail cell, she and 30 other people were ordered onto a bus and told not to look outside, she said. Seven hours later, she said, they arrived at a camp.

"At the camp, they put us in a cell. Then the next day, they ordered us out for what they said was exercise," the woman, who requested anonymity, told Al Jazeera.

"They beat us as they ordered us to exercise, and when we got tired, they continued to beat us. I tried to do what they said, but I couldn't, so they beat me more. Even when I was running, they were beating me all over my body."

Interrogation was carried out regularly to wear the detained down, the woman said.

"Five or six policemen interrogated each one of us every day. They kept threatening us. They said if you give false testimony, we will kill you."

Protests that started in November among people from the Oromo ethnic group have spread. Demonstrators from the Amhara region have also started to demand greater political and economic rights.

The Oromo and Amhara are the two biggest ethnic groups in Ethiopia. Both accuse the government of being dominated by members of the Tigrayan ethnic group, which makes up about six percent of the population.

Government promised accountability

Government leaders have said they communicate with opposition groups to listen to their grievances. They also promised that police found guilty of abuse will be held to account.

"We will do whatever it takes to make sure such things do not happen, and if they do happen, the people are not left unaccounted for," Getachew Reda, a government spokesman, told Al Jazeera.

Members of the opposition, though, say they have heard similar reassurances before.

In the latest bout of unrest earlier this week, protesters attacked foreign-owned businesses, according to the owners of a flower firm.

The Dutch company said crowds of people in the Oromia and Amhara regions torched flower farms as they targeted businesses with perceived links to the government. Flowers are one of the country's top exports.

Esmeralda Farms said its 10-million-euro investment ($11.1m) went up in smoke this week in Bahir Dar city and that several other horticulture companies were also affected.